General Principles - Landing — Page 284, Lesson 352

Let’s pick up right where we left off in the landing performance picture. We’ve already covered the basic forces at play during the landing roll, and now I want to walk you through two more critical factors that shape how much runway you actually need: air density and wind. First, density. The air density affects two things: the true airspeed for a given indicated airspeed, and the thrust or power of the engine. Now, during landing, thrust is small — the engines are essentially at idle. So the main effect of density is on the TAS. Remember, indicated airspeed is what the instruments show, but true airspeed is your actual speed through the air. When density is low — which happens with high temperature, low pressure, or high humidity — the TAS for a given IAS increases. That means you’re moving faster over the ground for the same indicated reading. The result is an increase in landing distance. But here’s the key qualifier: this effect is to a lesser degree than for take-off distance. So density matters for landing, but not as dramatically as it does on the take-off roll. Now wind. You’ll recall that a headwind decreases the true ground speed of the aeroplane for any given indicated airspeed. Let me unpack that. Ground speed is your speed over the landing surface. If you’re flying into a headwind, the air is moving against you, so for a given IAS, your forward speed over the runway is much less. As a result, the distance required to bring the aeroplane to rest is decreased. A tailwind has the opposite effect. It increases the ground speed for a given indicated airspeed, so your forward speed over the landing surface is much greater, and the distance required to bring the aeroplane to rest is increased. Now, here’s a critical operational recommendation. When you’re examining the effects of wind on landing performance, you should not use the actual wind that is given. Why? Just in case the wind changes to a worse condition than the one you planned for. So when calculating actual landing distances, you assume only 50% of the headwind component, and 150% of the tailwind component. Let me make sure that’s clear. If you have a 20-knot headwind, you only credit yourself with 10 knots of benefit. If you have a 20-knot tailwind, you penalise yourself with 30 knots of extra ground speed. This is a built-in safety margin. And most performance graphs that calculate landing distance already have the 50% headwind and 150% tailwind recommendations applied. So when you read a landing distance off a chart, that factor is already baked in. One more important note: there is no allowance for crosswinds, and therefore no safety factor for crosswinds. Crosswinds present an additional complication because of the effect of potentially crossed controls. Let me explain that. A crosswind tends to push the aeroplane off the centre line. To counteract that, you have to apply crossed controls — that means you’re holding aileron into the wind and opposite rudder to keep the aeroplane tracking straight. This creates additional drag and complicates the landing roll, but the performance calculations don’t give you any credit or margin for it. So you have to be aware that crosswind landings are inherently more demanding and the standard distance figures don’t account for that extra difficulty. So to summarise what we’ve covered: low density increases landing distance through higher TAS, but less than for take-off. Headwinds decrease landing distance, tailwinds increase it. You plan with only 50% of the headwind and 150% of the tailwind as a safety factor, and there’s no crosswind allowance at all. That’s the complete picture for these two factors.